1. Field of the Invention
The present invention relates to a semiconductive roller and a toner transport roller employing the same, as well as an electrophotographic apparatus including the toner transport roller.
2. Description of Related Art
Various types of image forming apparatuses utilizing electrophotography are increasingly improved, in order to satisfy requirements for speed increase, improvement in picture quality, colorization and downsizing.
The key to such improvements is toner. In other words, refinement of the toner, uniformization of the particle diameter of the toner, and sphericalization of the toner shape are necessary, in order to satisfy the requirements.
As to the refinement of the toner, fine toner having an average particle diameter of not more than 10 μm or not more than 5 μm has been developed. As to the sphericalization of the toner shape, toner having sphericity exceeding 99% has been developed.
In order to further improve the quality of formed images, polymerized toner is increasingly employed in place of the conventional pulverized toner. The polymerized toner exhibits extremely excellent dot reproducibility particularly in imaging of digital information, to enable formation of high-quality images.
A roller including a roller body made of a crosslinked substance of a semiconductive rubber composition prepared by blending a conductivity supplier such as carbon into a base polymer and a shaft made of a metal or the like inserted into the center of the roller body is generally employed as a developing roller for transporting charged toner to a surface of a photosensitive body and developing an electrostatic latent image formed on the surface into a toner image in an image forming apparatus.
In particular, a semiconductive roller having roller resistance adjusted to not more than 108Ω is suitably employed. The semiconductive roller can supply high chargeability to the toner in response to the refinement of the toner, the uniformization of the particle diameter of the toner and the sphericalization of the toner shape or the transition to the polymerized toner, and can efficiently transport the toner to the surface of the photosensitive body without adhering the same to the outer peripheral surface.
While the semiconductive roller must retain the roller resistance over the whole lifetime of the product, the durability of the existing semiconductive rollers is not sufficient for satisfying such a requirement.
In Patent Document 1 (Japanese Unexamined Patent Publication No. 2006-99036), for example, the type of rubber or carbon as the material for a base polymer is adjusted, to supply extremely high chargeability to a roller body of a semiconductive roller in an initial stage of manufacturing. Thus, improvement of initial performance (improvement in quality of an initial image) and retention of the performance (durability) are to be compatibly attained.
According to studies conducted by the inventor, however, only the quality of the initial image or only the durability can be improved at an extremely high level, while it is difficult to compatibly improve both of the quality of the initial image and the durability.
Patent Document 2 (Japanese Unexamined Patent Publication No. 2004-170845) discloses a semiconductive roller including a roller body made of a semiconductive rubber composition obtained by blending a base polymer prepared from ion-conductive rubber having uniform electric characteristics and a filler for adjusting a dielectric loss tangent and having a dielectric loss tangent set to 0.1 to 1.5.
Rubber, represented by epichlorohydrin rubber, containing chlorine atoms in the molecules or rubber containing an ethylene oxide monomer exhibiting ion conductivity as a copolymer component is used as the ion-conductive rubber.
However, the former rubber containing chlorine atoms generally has such high surface free energy that adhesiveness with respect to toner or an external additive such as silica added to the toner in order to improve fluidity or chargeability of the toner tends to increase. Also in the case of the latter rubber, the adhesiveness with respect to the toner or the like tends to increase due to increase in surface free energy.
According to Patent Document 2, further, an oxide film is formed on the outer peripheral surface of the roller body by ultraviolet irradiation or exposure to ozone. In this case, however, the oxygen concentration in the vicinity of the outer peripheral surface increases, and hence the adhesiveness with respect to the toner or the like tends to further increase due to the increase in surface free energy.
In addition, while the quantity of transportation of the toner can be reduced by improving the chargeability of the toner and hence a high-quality image such as a halftone image can be formed when the dielectric loss tangent is adjusted in the above range, the quantity of lamination of the toner is reduced on the outer peripheral surface of the roller body in this case, and hence the adhesiveness with respect to the toner or the like may further increase.
While adhesion of the toner or the like to the roller body does not much influence images formed in an extremely initial stage or continuously formed images, the influence is not negligible if images are formed under any of the following conditions (a) to (d), for example.
For example, while normally charged toner is transported to a reversely charged photosensitive body by electrostatic force (Coulomb force), the transportation of the toner with the electrostatic force is hindered if the adhesiveness of the roller body of the developing roller to the toner or the like is excessively high as described above. Therefore, the image density is reduced if images are formed under any of the following conditions (a) to (d), although the quantity of charge of the toner remains intact. In other words, the developing efficiency of the toner is reduced.
(a) Further images are formed after image formation is properly performed for forming about 2000 images of 1% in density, for example, and the toner relatively fits to the developing roller.
(b) The average particle diameter of the toner is not more than 8 μm, particularly not more than 6 μm.
(c) Images are not continuously formed but an image forming apparatus is temporarily stopped and subsequent images are formed on the next day.
(d) Images are formed in a low temperature and humidity environment, in which the quantity of charge of the toner is relatively increased.
The developing efficiency is particularly easily reduced in an image forming apparatus including a developing roller having a rotational speed set to not less than 20 rpm, for example, due to the speed increase.
When the developing efficiency is reduced, the quantity of toner not consumed by the development but repetitively circulating in a toner box is so increased that the toner is rapidly deteriorated to quicken reduction in the quantity of charge of the toner. Consequently, formed images are easily rendered defective due to the reduction in the quantity of charge.
The deterioration of the toner and the resulting reduction in the quantity of charge are regarded as quickened as the quantity of the transported toner is increased. In order to prevent such deterioration of the toner, the type and the quantity of a filler introduced into the semiconductive rubber composition forming the roller body of the developing roller may be adjusted, to reduce the adhesiveness with respect to the toner or the like and to improve the developing efficiency of the toner.
When the developing efficiency is improved by adjusting the type and the quantity of the filler, however, damage on the toner is rather increased, although the image density is improved. Therefore, the toner is deteriorated before the same is used up, and an image failure such as fogging (a phenomenon causing blackening of white portions of an image), in particular, is frequently caused immediately before the toner is used up.
Patent Document 3 (Japanese Unexamined Patent Publication No. 2005-225969) discloses a technique of preventing adhesion of toner or the like by adding wax to ion-conductive rubber thereby reducing surface free energy on the outer peripheral surface of a roller body. Patent Document 4 (Japanese Unexamined Patent Publication No. 2001-357735) discloses a technique of coating the surface of a conductive member with a treating agent having an amine compound for controlling chargeability of toner.
However, the wax or the amine compound is so easily transferred to the toner or a photosensitive body that the same may contaminate the toner or the photosensitive body, to reduce the quality of formed images. Further, the wax or the amine compound is gradually lost due to the transfer, and hence the effect of the wax or the amine compound cannot be retained over the whole lifetime of the product.